American International Journal of
Research in Formal, Applied
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ISSN (Print): 2328-3777, ISSN (Online): 2328-3785, ISSN (CD-ROM): 2328-3793
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Red Mud as Low Cost Adsorbent for Zn(II) ion – Kinetic, Thermodynamic
and Equilibrium Study
Sujata Kumar,1 Dhanesh Singh,2 Saroj Kumar 3
Lecturer, Kirodimal Institute of Technology Raigarh (C.G), INDIA
Associate Professor, K.Govt. Arts & Sc. College Raigarh (C.G), INDIA
Assistant Professor, K.Govt. Arts & Sc. College Raigarh (C.G), INDIA
Abstract: In the present study, red mud has been used as low cost adsorbent to remove Zn(II) ions from aqueous
solutions. The effect of different parameters such as initial Zn(II) ion concentration, temperature, pH and particle
size have been studied. For kinetic study, Lagergren first order equation and pseudo second order equation have
been used. For equilibrium study, Langmuir equation as well as Freudlich equation have been discussed .
Thermodynamic parameters such as Gibbs free energy change,entropy change and enthalpy change have been
calculated and discussed.
Key words : Red mud, Zn(II) ion, Langmuir isotherm, Freundlich equation, Lagergren first-order equation, pseudosecond- order equation.
I. Introduction
Zinc is used in industries like galvanization, diecasting, plastic, paints, cosmetics etc. From these industries, the
effluents going to the nearby river pollute water and soil and cause severe health problems. A number of methods
such as ion-exchange,reverse osmosis,membrane filtration etc. have been reported to remove the metals from
polluted water. However, either these methods are costly or having insufficiency of technique. Among various
methods, adsorption method has been of much interest to scientists in recent past, since it is both effective and easy
to handle. A large number of substances have been studied as adsorbent [1]-[4]. In the present study, red mud , a
waste by-product of aluminium industry has been used as adsorbent.
II. Material and Methods
Red mud was obtained from BALCO, Korba (C.G., India). It is alkaline in nature and so was washed till
neutral,dried at 105oC and sieved. SEM, FTIR and XRF analysis were obtained from IIT-Bombay to characterise it.
A.R. quality Zn(NO3)2 was used to prepare the stock solution.
Batch mode experiments were carried out for the study by shaking 1.0 g of red mud with 25 mL aqueous solution of
Zn(II) of given concentration in different glass bottles. After pre-determined time interval, the solution was
centrifused and filtered and the solution was analyzed for concentration of Zn(II) ion by Systronic
Spectrophotometer 118 model. Various parameters were contact time (20,40,60,80,100,120 and 140 min.), pH (2.0,
4.0, 6.5 and 8.0), temperature (303K, 313K and 323K) and particle size (45µ,75µ and 150µ). Initial Zn(II)
concentration used were 25,50,75,100,125,150,175, 200, 225 and 250 mgL-1 for the equilibrium study and for rate
study it was 100,150, 200 and 250 mgL-1.
III. Results and Discussion
Characterisation of red mud
Red mud obtained from different sources contain the same basic chemical elements but in different
proportions. Chemical composition of the present red mud obtained from XRF studies are : SiO2 (43.17%),
Al2O3(13.25%), Fe203(41.20%), CaO(1.09%), MgO(0.73%) and TiO2(1.26%).
The FTIR spectra of red mud before and after adsorption is shown in figure-1(a) and 1(b). It shows a broad band
around 3500 cm-1 , which is attributed to surface -OH group of silanol groups ( -Si-OH) and adsorbed water
molecules on the surface. A peak around 1400 cm-1 – 1600 cm-1 is attributed to presence of carbonate. A strong peak
at 995.22 cm-1 is due to stretching vibration of Si(Al)-O group[5]. New peaks obtained are due to zinc and confirm
the adsorption.

Effect of initial Zn(II) ion concentration and Contact time
The relationship between amount adsorbed (mgg-1) andPage
time(min.)
at different initial concentration has been shown
1/1
in figure 3. It is evident from the graph that the amount adsorbed increases with time till equilibrium is reached. The
time of equilibrium is independent of initial concentration. Initially the rate of adsorption is fast which might be due
to the presence of more number of active sites on the surface of red mud. As the process of adsorption goes on, the
number of active sites decreases and so the rate of adsorption also decreases[6] –[7].

Effect of pH
pH of the medium has pronounced effect on the adsorption. From figure-4 it is evident that the amount of Zn(II)
adsorbed on red mud increased from 1.64 mgg-1( 65.6 %) to 2.38 mgg-1 (95.2 %) by increasing pH of solution from
2.0 to 8.0.
Speciation studies [8] have shown that at low pH cadmium remains in the form of Zn++ and at higher pH in the
form of Zn(OH)2. It is probable that in acidic medium positively charged surface of adsorbent does not favour the

association of cationic adsorbate species. In alkaline medium negatively charged surface offers the suitable sites for
the adsorption of Zn++ and Zn(OH)2 .
Effect of temperature
The relationship between qe(mgg-1) and time(min.) at different temperatures has been presented in figure 5. It is
evident that adsorption of Zn(II) ion on red mud increases from 2.03mgg-1 (81.2 %) to 2.29 mgg-1(91.6 %) by
increasing temperature from 303K to 323K indicating the process to be endothermic The rate constant of adsorption
are 3.89x 10-2, and 4.58x 10-2 per min at 303K and 323K respectively which indicate that the rate of adsorption also
increases with temperature.
3
3
303K

The Freundlich equation [10] has also been used for the adsorption of zinc (II) on red mud which is represented as
logqe = log Kf + 1/n log Ce
where qe is the amount of Zn(II) ion adsorbed (mgg-1), Ce is the equilibrium
concentration of Zn(II) ion in solution(mgL-1) and Kf and n are constants for the adsorption capacity and intensity of
adsorption respectively.
Plots of Ce/qe vs Ce for Langmuir isotherm and of logqe vs logCe for Freundlich isotherm have been given in figure-7
and 8 respectively. Different parameters obtained have been given in table-1.
It is evident from the graphs and table that R2 value obtained for Langmuir model is higher than that of Freundlich
and so it may be concluded that data fits better in Langmuir adsorption isotherm. It can also be seen that adsorption
capacity φ increases with temperature.

Kinetics of Adsorption
The Lagergren first order[11] and pseudo-second-order[12] have been used to discuss the adsorption kinetics.
The Lagergren first order rate equation is represented as :
log (qe – qt) = log qe – k1.t/2.303
Pseudo second order rate equation is represented as :
t/qt = 1/k2.qe2 + t/qt
where qe and qt are the amounts of Zn(II) adsorbed (mgg-1) at equilibrium and at time t , respectively. k1 is the
Lagergren rate constant (min-1). k2 (g/mg/min.) is the rate constant of second order adsorption. Plots for both
Lagergren first order and pseudo second order equation has been shown in figure -9 and 10 respectively.
Different adsorption parameters for both models have been presented in table – 2. Values of qe(cal) and k1 and k2 at
different initial concentrations have been calculated from the slope and intercept respectively . These values have
been given in table- 2.
80
0.0
100 mgL-1

It is evident from table- 2 that R2 values are higher in the case of pseudo second order equation than in the case of
first order equation. It may be concluded therefore that kinetic data fit better in pseudo second order equation. It can
be seen from the table that k2 decreases with increase in concentration. It might be due to the possibility of low
competition for active sites at lower concentration of metal ions. At higher concentration of metal ions, the
competition for the surface active sites increases which decreases the rate. Other investigators have found the same
result [13].
Thermodynamic treatment of the adsorption process
The thermodynamic parameters such as free energy, enthalpy and entropy changes have been calculated using the
following equations [14].
Kc = Cs/Ce
∆G = - RT ln Kc
log Kc = ∆S/2.303 R - ∆H/2.303 RT
where Ce is the equilibrium concentration in solution in mgL-1 and Cs is the equilibrium concentration on the
adsorbent in mgL-1 and Kc is the equilibrium constant. The Gibbs free energy, ∆G was calculated from the above
equation. Slope and intercept of the straight line obtained from the plot between log kc vs 1/T (not shown) gives the
value of ΔH and ΔS respectively . These values have been given in Table- 3.
The values of activation energy (Ea) and sticking probability (S*) have been calculated from the experimental data
using modified Arrhenius type equation related to surface coverage(θ) as follows [15]
θ = ( 1- Ce/Ci)
S* = (1- θ)e -Ea/RT
The sticking probability , S* , is a function of the adsorbate/adsorbent system under consideration, depending on
temperature and should satisfy the condition 0<S*<1 .The values of Ea and S* has been calculated from slope and
intercept of the plot of ln(1-θ) versus 1/T shown in figure-15 respectively and have been given in Table-3.

Temp. K
∆G , kJ/mol
∆H , kJ/mol
∆S , J/mol
Ea , kJ/mol
S*, J K mol-1
303
-3.686
37.68
136.62
32.78
4.20X10-07
313
-5.088
323
-6.417
The negative values of free energy indicates the spontaneity of the process. Positive ∆H value shows the
endothermic nature of adsorption. The positive value of ∆S shows the affinity of the adsorbent for the Zn(II) ions.
The endothermic nature of the adsorption process is supported by positive value of Ea which is in accordance with
the positive values of ∆H. Since S*<<1, it indicates that the probability to stick on surface of red mud is very high
[16].
Mechanism
Speciation [17] of Zn(II) with varying pH has been shown in figure-11.

Figure-11. Speciation of Zn(II) with varying pH
It is evident that at lower pH , zinc is in the form of Zn+2 and at higher pH it is in the form of Zn(OH)2 . It is
probable that in acidic medium positively charged surface of adsorbent does not favour the association of cationic
adsorbate species. In alkaline medium negatively charged surface offers the suitable sites for the adsorption of Zn+2
species [18].
OHM – OH
----------→ MO+2
MO + Zn
----------→ MOZn+
+
MO + Zn(OH)
----------→ MOZn(OH)
where M represents the adsorbent sites on surface.
IV. Conclusion
It is evident that initial Zn(II) ion concentration, contact time, pH and temperature have marked effect on adsorption.
The equilibrium data are best explained by Langmuir adsorption isotherm. Kinetics of adsorption follows second
order rate equation. Thermodynamic parameters also favour the adsorption. It is expected that red mud may be used
as an efficient adsorbent under suitable conditions.
Acknowledgement
.We are thankful to SAIF, IIT Bombay, for SEM and FTIR analysis of red mud.